In recent years, computer makers have been increasing the number of operating frequency of CPU, the control center of a computer to allow CPU to transmit enormous amounts of control signals in a short time and to shorten the controlling time thereby to raise the data processing speed. For example, operating frequency of CPU was 1 GHz in 2001. In 2002 operating frequency became 2 GHz and in 2003 operating frequency is about to reach 3 GHz. As operating frequency of CPU increased, however, the problem of return noise demanding solution surfaced.
In voltage wave and current wave, generally there exist an incident wave propagating in the positive direction and a reflected wave propagating in the reverse direction at the same time in case a load impedance Zr is connected to a receiving end 104 of a line 100 of a finite length of a length 1 from a sending end 102 as shown in FIG. 1.
If the characteristic impedance of the line 100 is ZO and assuming the voltage upon a signal arriving at the receiving end 104 to be Va and the reflected voltage generated at the receiving end 104 to be Vr, the following will hold true. Vr/Va=(Zr−ZO)/(Zr+ZO)=m (1)
where m is a reflection coefficient at the receiving end. The reflected wave of the receiving end 104 returns to the sending end 102 and reflects at the sending end again and moves toward the receiving end 104. As long as the reflection coefficient m is not 0, the reflected wave of the receiving end 104 repeats the above.
FIG. 2 shows the change caused by reflection of voltage Va of the sending end 102 and voltage Vr of the receiving end 104 shown in FIG. 1. If transmission time is made to be t0, the signal arrives at the receiving end 104 with the propagation delay time of the line 100. In this situation, the voltage Vr of the receiving end 104 is the sum of the voltage Va of the incident wave from the sending end 102 and the voltage of the reflected wave.
The reflected wave at this time also arrives at the sending end 102 falling behind just the same td with the propagation time delay of the wiring and reflects. The reflected wave is capable of becoming a negative by impedance; therefore, as the reflected wave repeats a reflected wave, the voltage Vr of the receiving end 104 subsides and settles at the voltage Va sent from the sending end 102.
In general, the load low in the response level is also used for the load Zr at the receiving end 104 when transmission signal frequency is low. In other words, as shown in FIG. 3B and FIG. 4B, reflection is repeated before the sensitivity of the load becomes valid at time t1 as shown in FIG. 3A. Then either the voltage Vr at the receiving end settles down to a constant voltage or during the rise time of the signal up to time t1 as shown FIG. 4A, the reflection of the voltage Vr settles down and becomes constant. Therefore, the load does not make an error.
However, when operating frequency of CPU is made high, the load with high sensitivity is used to make the load compatible with CPU. Accordingly, the width of a signal determined by a clock also becomes shorter. FIG. 5A illustrates repetition of reflection. When operating frequency of CPU is high, the load responds to the reflection and acts erratically as shown in FIG. 5B.
The voltage at the receiving end changes as shown in FIG. 5A in case “111” were sent from the sending end, for example. The load makes error of judging the “111” as “101.” In recent years operating frequency exceeded GHz. To make such a CPU operate normally, it became important to solve the problem of return noise of transmission waveforms.
Methods to analyze and confirm signal waveforms of the receiving end using 3-D electromagnetic analysis exist. In those methods random signals consisted of bit streams of about 100 bits are input for analysis and confirmation of waveforms. Among those methods, eye pattern analysis is most widely known.
A problem of signal transmission in transmission path of a circuit board carrying CPU is that return noise is invariably generated in a transmission waveform. Depending on the row of the signal of 1 and 0, return noise overlaps and sometimes the overlapping reaches the level at which an error occurs.
In eye pattern analysis a row of random signals consisted of bit streams of about 100 bits are to be made an input waveform. The signal waveform obtained at the receiving end from the signal transmission is superimposed bit-by-bit and displayed. On the display, the signal waveform is made in the length of 1 bit. In this way, eye pattern analysis makes it easy to observe the quality of the transmission signals and to differentiate a bad transmission waveform from a good transmission waveform.    Patent Document 1: EP 1083501 A1    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2000-35984    Patent Document 3: U.S. 2002/0032555 A1    Patent Document 4: Japanese Patent Application    Laid-Open Publication No. 2002-163320EP